1. Field of the Invention
The present invention relates to the field of liquid crystal displays (LCDs), and more particularly, to an LCD device and a transforming circuit thereof.
2. Description of the Prior Art
An LCD device usually comprises an LCD panel and a backlight system. Light produced by the backlight system is transmitted through the LCD panel, which forms images on the LCD panel. The transmission of light is controlled by an orientation of liquid crystal molecules in the LCD panel. The backlight system comprises a light source and a backlight driving circuit. The zoom multiple of the backlight driving circuit is restricted to the maximum duty cycle of a chip, which makes it necessary to increase the zoom multiple of a transforming circuit.
Referring to FIG. 1 showing a conventional transforming circuit which drives a light-emitting diode (LED) light source in the backlight system, the transforming circuit comprises a first coil 110, a second coil 120, and a switch unit 130.
One terminal 111 of the first coil 110 is used for being connected to input voltage. The other terminal 113 of the first coil 110 is used for being connected to one terminal 121 of the second coil 120. The other terminal 123 of the second coil 120 is used for outputting transformed voltage. A controlling terminal 131 of the switch unit 130 is used for inputting a driving signal. A first terminal 133 of the switch unit 130 is used for being grounded. A second terminal 135 of the switch unit 130 is used for being connected to the other terminal 113 of the first coil 110 and to a common terminal of the one terminal 121 of the second coil 120.
When the driving signal causes the switch unit 130 to conduct, voltage applied to the one terminal 111 of the first coil 110 is equal to the input voltage. Voltage applied to the other terminal 113 of the first coil 110 is zero. At this point, the first coil 110 stores energy because of the input voltage. Meanwhile, because of the coupling effect of the first coil 110 and the second coil 120, voltage applied to the other terminal 123 (the output terminal) of the second coil 120 is −N times of the input voltage in which “N” indicates a turns ratio of the second coil 120 to the first coil 110.
The driving signal causes the switch unit 130 to be turned off. Assuming that the voltage applied to the other terminal 113 of the first coil 110 is defined as Vd, Vin*Ton=(Vd−Vin)(T−Ton) is satisfied based on the volt-second balance principle in which T indicates a switching period of the switch unit 130, Ton indicates conduction time, and Vin indicates input voltage. Vd=Vin/(1−D) is derived in which D=Ton/T is satisfied. At this point, the voltage applied to the other terminal 113 of the first coil 110 is higher than the voltage applied to the one terminal 111 of the first coil 110. The voltage drop between the two terminals 113 and 111 is Vd−Vin=Vin*D/(1−D). According to the principle of transformation, the voltage drop between the one terminal 121 of the second coil 120 and the other terminal 123 of the second coil 120 is N*Vin*D/(1−D). Moreover, the voltage applied to the other terminal 123 is higher than the voltage applied to the one terminal 121. The voltage applied to the one terminal 121 is defined as Vd, so the voltage applied to the other terminal 123 is defined as Vo=Vin*(1+N*D)/(1−D) in which Vo indicates the voltage applied to the other terminal 123.
However, according to the aforementioned, very high reverse transformed voltage would be output from the other terminal 123 of the second coil 120 when the driving signal forces the switch unit 130 to conduct, i.e., in a non-working state. Due to this reason, a rear-stage circuit needs to have a very high negative-pressure resistance. At this point, the electric current in the second coil 120 would flow into the ground through the switch unit 130 when the switch unit 130 conducts. As a result, loss of energy in the circuit would be inevitable, lowering the effective power in the transforming circuit.